Graham R. Webster

Optical studies of electric fields in poly(2-methoxy-5-ethyl(2(')-hexyloxy) para-phenylene vinylene) light-emitting diodes

We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade.

Investigating the effect of conjugation in MEH-PPV

Solution state absorption and fluorescence of samples of fully and partially conjugated MEH-PPV at 77K and 300K have been measured to probe the distribution of conjugated segments within the polymer. The absorption of the partially conjugated polymer is at shorter wavelength and has a larger full width at half maximum at all temperatures than that of the fully conjugated polymer. The spectra are analysed by considering the polymer chain to be made up of a number of non interacting conjugated segments. Using this approach it is possible to explain all the features seen in the spectra.

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO2 microstructure, time and illumination power.

Nanocomposite titanium dioxide/polymer photovoltaic cells have been fabricated using poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEHPPV). Two different types of titanium dioxide were used, one synthesized using a sol-gel method, the other was a commercial paste. The crystal structure, porosity and absorption spectra of the titanium dioxide layers were measured, and the titanium dioxide synthesized using the sol-gel method had a much lower level of anatase. The photovoltaic properties of the ITO/TiO2/MEHPPV/Au cells, which were similar for both types of TiO2, were measured as a function of illumination power and compared with equivalent circuit models. A simple equivalent circuit model incorporating a diode, two resistances and a light induced current was inconsistent with the illumination -- dependent data and was improved by adding an illumination dependent shunt resistance. A very long lived, photo-induced increase in dark current was observed, which could not be explained by a polymer degradation mechanism or an increase in temperature under illumination, but was more likely to be due to trapped charge.

Optical studies of polymer light-emitting diodes using electroabsorption measurements

Abstract We have studied the electrical fields in single-layer polymer light-emitting diodes (LEDs) using electroabsorption (EA) spectroscopy. The materials under investigation were PPV derivatives, namely poly(2,5-dialkoxy- para -phenylene vinylene) and a precursor route poly(2-methoxy-5-ethyl(2′-hexyloxy)- para -phenylene vinylene). The applied bias generates an opposing electric field that decays over time. The magnitude of the reversed field increases as the applied DC bias is increased and can be as high as one-third of the applied external field. The development of the internal field is less pronounced in vacuum than in air and it vanishes for devices prepared and tested under inert conditions.

Charge injection into OLED's during operation studied by Electroabsorption screening

Abstract Organic light emitting devices (OLED) were operated using an AC voltage with DC offset (operating voltage). Electroabsorption (EA) signals as a function of a DC-offset were measured at 2f. Higher absolute DC offset voltages lead to a reduction of the EA signal. The effect observed can well be explained in terms of an extended electroabsorption model including Thomas-Fermi screening: The AC component of the electric field is shielded due to charge injection caused by the DC offset This new device characterization technique allows monitoring the charge carrier statistics in the device during operation. It was found that the EA signal indicates the threshold of monopolar and bipolar charge injection. The presence of negative charges enhances drastically the injection and presence of positive charges in the device,

Twofold efficiency increase in nanocrystalline-TiO2/polymer photovoltaic devices by interfacial modification with a lithium salt

Modification of the interface of titanium dioxide/poly[2-(2-ethylhexyloxy)-5-methoxy-1,4,-phenylenevinylene] (TiO2/MEH-PPV) nanocomposite photovoltaic devices with a lithium salt, Li[CF3SO2]2N, is shown to result in a twofold increase in device efficiency. The devices are of the type ITO/TiO2/MEH-PPV/Au. The TiO2 layer is deposited by doctor blading a colloidal anatase paste, and the polymer is then spin-coated on top followed by thermal evaporation of gold contacts. Careful control of manufacturing conditions and use of a 35 nm polymer layer leads to a device efficiency of 0.48% for un-modified devices. The increased efficiency following Li treatment is the result of a 40% increase in both the short-circuit current and fill factor, while the open-circuit voltage remains unchanged. A maximum efficiency of 1.05% has been achieved under 80% sun illumination. This represents a record efficiency for this type of cell. Photoconductivity experiments show a substantial increase in conductivity of the TiO2 layer following Li modification. Interfacial modification is done via a simple soaking procedure, and the effect of varying the concentration of Li[CF3SO2]2N is discussed. We report investigations into optimization and the mechanism of such improvement, for example by varying processing parameters of the modification procedure or the ionic species themselves.

Non-Steady State Operation of Polymer/TiO 2 Photovoltaic Devices

We present data on the initial period of operation of Gilch-route MEH-PPV/TiO2 composite solar cells (CSCs) which show that during this period the CSCs operate in a non-steady state regime. The behavior is complex and may include a gradual rise of the open circuit voltage (Voc) and of the short-circuit current density (Jsc) with time, a passage through a maximum of either or both parameters, and even a sign reversal. The mechanisms most probably contributing to the transient processes are: i) diffusion driven redistribution of charges resulting in the build up of a quasi steady state charge density profile across the device; ii) photo-doping resulting in a relatively slow increase of the average charge carrier concentration and consequently of the conductivity of the device. The latter is responsible for a strong decrease in Voc, and is evidenced by the significant increase in dark current after device illumination.

Electrode surface structures for studying long-range electron transfer in biological systems.

Gold electrodes modified with a self-assembled monolayer (SAM) of organo-thiol present a well-defined electrode surface for the adsorption of protein molecules to be studied by protein film voltammetry (PFV). Following the publication of the synthesis and characterization of ferrocene-terminated oligophenylenevinylene (OPV) thiol monolayers on gold by Chidsey and coworkers, ethyl-terminated OPV “molecular wires” have been synthesized and incorporated into SAMs on gold. The study of a model system, by PFV, using the simple electron transfer protein, azurin, has demonstrated that OPV SAMs facilitate fast electron transfer between the protein and the electrode. This concept is being extended to develop stable OPV electrode surface structures, which mimic biological membranes and are capable of transferring electrons over distances greater than 30 Angstroms with the goal of achieving fast electron exchange with large and complex membrane-bound redox proteins.